a. Field of the invention
The present invention relates to an oil immersion liquid for use with fluorescence microscopes.
B. Description of the prior art
Objectives designed for a magnification of x100 and a numerical aperture N.A. of 1.25 or higher are required for studying chromosomes, etc. by using fluorescence microscopes. For this purpose, oil immersion type objectives are adopted for this type of fluorescence microscopes. However, fluorescence emitted from specimens to be observed is generally at a low intensity and satisfactory observation is impossible when images of specimens to be observed are not formed with high contrast. It is, therefore, impossible to satisfactorily observe images of fluorescent specimens when image contrast is degraded due to flare produced by the lens system or auto fluorescence emitted from a substance existing in the lens system or related components.
Conventionally, cargile oil and cedar oil are used as oil immersion liquids for ordinary microscopes but these oils emit auto fluorescence.
Further, distilled water which does not emit auto fluorescence, glycerin which emits weak auto fluorescence but scarcely affects fluorescence microscopy and cargile oil which emits relatively weak auto fluorescence are also used as oil immersion liquids for fluorescence microscopy. Of the above-mentioned oil immersion liquids, cargile oil emits certain degrees of auto fluorescence which unavoidably produces flare and affects fluorescence microscopic observations. Distilled water does not emit auto fluorescence and is deal for fluorescence microscopy in this respect. However, aberrations are corrected for cargile oil in general oil immersion type objectives, while distilled water has a refractive index (Nd = 1.333) which is largely different from that of cargile oil (Nd = 1.514). Therefore, distilled water will generally aggravates aberrations and, in addition, degrades resolution and image contrast, making it impossible to satisfactorily observe images of fluorescent specimens. As a result, it is required to use an objective specially designed for use with distilled water in order to use it as an oil immersion liquid for fluorescence microscopy. In addition, distilled water has another drawback that it has a surface tension which is too low to select a long working distance when it is used as an oil immersion liquid. Glycerin scarcely affects fluorescence microscopy with its auto fluorescence as described above and has a refractive index of 1.458 which is close to that of cargile oil, therefore being usable with objectives designed for use with cargile oil simply by slightly modifying the objectives. However, glycerin is moisture-absorptive and its refractive index decreases close down to that of distilled water as time elapses. This decrease in refractive index of glycerin degrades both resolution and image contrast similarly to the case where distilled water is used as an oil immersion liquid.